In general, the compression-refrigerating cycle of a refrigerator, such as a compression-type refrigerator composed of a compressor, a condenser, an expansion valve, and an evaporator, is constructed to allow a mixture of a refrigerant and a lubricating oil to circulate in this closed system. In such a compression-type refrigerator, conventionally, chlorofluorocarbon such as dichlorodifluoromethane (R-12) or chlorodifluoromethane (R-22) has been used as a refrigerant. For using in combination with chlorofluorocarbon, various kinds of lubricating oils have been produced and employed. However, there is a concern that the chlorofluorocarbon compounds, which have been conventionally used as refrigerants, may cause environmental pollution problems, such as ozone layer destruction, when they are discharged into the atmosphere. For this reason, hydrofluorocarbon, fluorocarbon, or the like, as typified by 1,1,1,2-tetrafluoroethane (R-134a), has been used as a new refrigerant. Such the hydrofluorocarbon or the like may not destroy the ozone layer but its prolonged life in the atmosphere may affect on global warming. In recent years, therefore, natural refrigerants without such the problems are considered to be used.
On the other hand, in a refrigerator oil composition, a phosphorous additive, typically tricresyl phosphate (TCP), has been used as an extreme-pressure agent. However, such a phosphorous extreme-pressure agent had a problem in that it could be denatured and become sludge by thermal decomposition or hydrolysis, or by a tribochemical reaction or the like on a frictional surface. In addition, by denaturing and sludging, the phosphorous additive would be exhausted so that it could not retain its lubrication property for a long period of time, thereby resulting in seizure or wear.
Among the conventional lubricating oil compositions for refrigerators using flon-based refrigerants, a sulfur-containing lubricating oil added with an organic sulfur compound has been proposed (Patent Document 1). The sulfur-containing lubricating oil proposed is excellent in thermal resistance and wear resistance with small variations in viscosity against thermal changes, as well as excellent in flon solubility. According to Patent Document 1, in the flon-based lubricating oil composition for a refrigerator, the content of an organic sulfur compound is important. For example, when mineral oil is used as a base oil, the organic sulfur compound used may be naturally found in mineral oil. However, lubricating-oil compositions for a refrigerator using natural refrigerants have not been provided with sufficient wear resistance and extreme-pressure property only by controlling their sulfur contents.
Patent Document 1: Japanese Patent Application Laid-Open No. 58-103594